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MODE
MODE
8 µs
P1 P2 P3
P1
Interrogations (1030 MHz)
P2
21 µs
P3
Aircraft responds
with Identity (Mode A) Code
Aircraft responds
with altitude (Mode C) data
Figure 3-7: 1030MHz interrogation signals
In reply, the aircraft transponder sends a 12-bit burst of data (each pulse separated by
1.45 µs).
3.3.3.3 Development of SSR
Secondary Surveillance Radars have increased in complexity over their design lifetime.
SSR grew out of the military wartime system known as IFF (Interrogate Friend or Foe).
With IFF, a target picked up by primary radar would subsequently be interrogated by the
secondary radar and, if it did not give the appropriate response, it would be assumed to
be hostile. With SSR, the principle is similar except that the transponder may return either
identity (Mode A) or height (Mode C) information and an unresponsive target is not
necessarily assumed to be hostile. [Note that the SSR system also includes some typical
military identification modes: Mode 1, 2, 3 and 4, where Mode 3 is analogous to Mode A].
Mode A identity codes suffer from the limitation that only 4096 combinations are possible
using the pulses available (octal 0000-7777). In modern European airspace, that number
can prove inadequate; as several aircraft may be assigned the same identity code (their
origin may be in geographically separated airspace regions, but as the flight progresses,
two or more of these aircraft may appear via the same SSR). An optional Special Position
Indicator pulse (SPI) is only set if the pilot activates the IDENT key on his control panel
(where a controller needs to differentiate aircraft with the same Mode A code). The SPI
pulse is recognised by ground systems, and an indication is made on the controller
display.
Original SSR systems used a technique known as ‘sliding window’ to estimate the
azimuth of the target. Sliding window interrogators repeatedly send out interrogations,
and the aircraft’s azimuth can be pin-pointed as the mid-point between the leading and
trailing edges of the reply (i.e. when it started and stopped responding). This technique’s
accuracy relies on receiving a relatively high number of valid replies to confirm the
existence of a target.
More modern SSR interrogators use a monopulse technique, utilising a pair of differential
receiving antennas, as shown in Figure 3-8 below. Both antennas receive the reply, and
by amplitude or phase measurements the azimuth can be calculated. By comparing the
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